Multistage thermostats are sometimes used to control HVAC systems equipped with a primary stage that can be used for heating or cooling during periods of normal operation and one or more auxiliary stages that can be used for heating or cooling during periods of high demand. In some residential heating systems, for example, such multistage thermostats can be used to control a heat pump having a compressor that operates as the primary heating stage during normal operation and an auxiliary stage heater that can be activated during periods of peak-demand for secondary heating. Such auxiliary stage heating may occur, for example, in response to a step change increase in the temperature setpoint entered by a user, or when the heat pump is unable to supply adequate heat to maintain the building at a certain temperature.
Typically, the auxiliary stage heater will comprise an electric-based component that is more expensive to operate than the primary stage component, which is usually gas powered or a higher-efficiency electric system. Some examples of common auxiliary heating sources can include resistance heater strips, electric-powered furnaces, fossil fuel furnaces (e.g. fuel oil, propane or natural gas furnaces), as well as other sources. Although often capable of quickly generating heat, such auxiliary stage sources typically consume more power than the primary stage source.
For many multistage thermostats, a step change increase in temperature setpoint typically creates an increased demand on the HVAC system, causing the thermostat to turn on the auxiliary stage either immediately when the new setpoint is received from the user, or at a later time once a predetermined period of time has elapsed without achieving the new setpoint. While activating the auxiliary stage results in achieving the new setpoint more quickly, such activation increases energy usage and is thus less efficient. A tradeoff therefore exists in many multistage HVAC systems between comfort and energy conservation.